The present invention relates to injection devices for injecting medicament and other types of fluids to be used in for example cosmetic treatment.
This type of drugs often have a high viscosity which means that they require high forces in order to press the fluid through a hollow needle when injecting them.
Auto-injectors having an automated injection function often work with spirally wound compression springs, where the springs act on a plunger rod, which in turn acts on a stopper inside a medicament container, for expelling medicament through an attached needle. The other end of the spring is often abutting an inner end surface of the housing of the device, which means that the housing has to be dimensioned to the forces of the spring.
When handling fluids with high viscosity this becomes even more pronounced because of the high forces required to expel the medicament. Further the spring becomes very large both regarding the diameter of the wound spring and also the diameter of the thread of the wire. The size of the spring means that the device becomes large, and for some applications and customers, such sizes of the devices are not acceptable.
Document WO 9421316 discloses an injection device arranged with two coil springs of different diameter working in concert. The idea of having two springs is to handle the rather long travel of the drive member, which could be difficult with a single spring because of a tendency of buckling when fully compressed and becoming entangled with interior components of the device.
With the dual spring arrangement one spring is arranged between an end plate and the base of an elongated cup. The cup has also an outwardly flange and the second spring acts between this and a wall across the cylindrical drive member.
This design can be used for handling long travel but cannot be utilised when larger forces are required. It is not possible to add the forces of the two springs because of the intermediary cup. The rear spring acts to move the cup forward, and thus the front spring, and then the front spring should act on the drive member. This means that the rear spring has to be so strong that the cup will not be moved backward due to the force of the front spring.